How do you handle date and time attributes in role playing dimensions?

Role playing dimensions are a common feature of dimensional modeling, a technique for designing data warehouses and business intelligence systems. They allow you to use the same dimension table for different purposes in a fact table, depending on the context of the analysis. For example, a date dimension can be used to filter, group, or calculate measures based on order date, ship date, or delivery date. But how do you handle date and time attributes in role playing dimensions? In this article, you will learn some best practices and tips to deal with this challenge.

1 Use surrogate keys

A surrogate key is a unique, artificial identifier that is assigned to each row in a dimension table, regardless of the natural or business keys. Surrogate keys are essential for role playing dimensions, because they allow you to join the same dimension table multiple times to the fact table, without creating ambiguity or duplication. For example, you can use a surrogate key called order_date_key to join the date dimension to the fact table based on the order date, and another surrogate key called ship_date_key to join the same date dimension based on the ship date.

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Shubha D.

Director/Enterprise Data Management Leader and Data SME
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Surrogate keys are a great way to manage your data in your dimensions and ensure uniqueness and performance. However, I would probably go with the natural date key as there no compelling technical argument or performance increase to use the surrogate key for time. You would have to covert the date keys & if you are performing arithmetic operations, you are better off performing it on the date. You can get different results applying on an int versus date. It also depends on the database. DB2. uses 4 bytes for date, Oracle stores it as no. If your granularity is lower than a day then surrogate would be better. However, I would keep time of day separate from the date dimension & separate your timezone too

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2 Separate date and time

Another best practice is to separate date and time attributes into different columns or tables, depending on the level of granularity and analysis required. This way, you can avoid unnecessary complexity and confusion when dealing with role playing dimensions. For example, you can have a date dimension that contains only date-related attributes, such as year, month, day, quarter, week, etc., and a time dimension that contains only time-related attributes, such as hour, minute, second, AM/PM, etc. Then, you can use surrogate keys to join the date and time dimensions to the fact table as needed.

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3 Use views or aliases

A view is a virtual table that is defined by a query, and an alias is a temporary name that is assigned to a table or a column in a query. Both views and aliases can help you simplify and clarify the role playing dimensions in your dimensional model, by giving them meaningful and descriptive names. For example, you can create a view or an alias called order_date that refers to the date dimension joined to the fact table by the order_date_key, and another view or an alias called ship_date that refers to the same date dimension joined by the ship_date_key. This way, you can easily distinguish and use the role playing dimensions in your queries and reports.

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4 Consider time zones and calendars

A final tip is to consider the impact of time zones and calendars on your role playing dimensions, especially if you are dealing with global or regional data. Time zones and calendars can affect how you define and interpret date and time attributes, such as the start and end of a day, week, month, quarter, or year. For example, you may need to adjust the date and time attributes based on the local time zone of the source system, the target system, or the user. You may also need to account for different calendar systems, such as fiscal, lunar, or religious calendars, that may have different rules and conventions for date and time calculations.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Shubha D.

Director/Enterprise Data Management Leader and Data SME
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It also depends on what questions or how you would like to use the attribute and adjust the model based on that. Follow the 80-20 rule. You want to keep it simple and make it easy for querying

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Kurt Cagle

Editor In Chief @ The Cagle Report | AI, Data Modeling
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One way of modeling temporal sequences is to avoid time notation altogether in favor of duration intervals relative to an arbitrary starting point. This notation, using ISO duration notation, uses the string PnYnMnDTnHNnSn.mmmm to indicates years down microsecond. In the model, you can then define the starting point as needed with a dateTime at some later time. This is very useful when you have events that are playing out in real time, have ambiguous starting points (such as is often the case with novels or movies), or cross time zones. Note that this points out one (of many) distinctions between relational DBMs and knowledge graphs. Decomposing time as shown in the article is highly wasteful of memory and slow.

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How do you handle date and time attributes in role playing dimensions?

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1 Use surrogate keys

2 Separate date and time

3 Use views or aliases

4 Consider time zones and calendars

5 Here’s what else to consider

Dimensional Modeling

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